Not Applicable.
1. Field of the Invention
This invention relates to implantable bone prostheses, and more particularly to joint prostheses that attach to bone and have variable bone affixation rates and final strengths at different locations on the prosthesis.
2. Description of the Related Art
The replacement of joints, such as the shoulder, hip, knee, ankle and wrist, with prosthetic implants has become widespread. The size, shape and materials of a joint prosthesis affect the extent of bone growth into and surrounding the prosthesis, both of which contribute to fixation of the prosthesis within the patient""s bone.
Joint prostheses may be affixed to or within natural bone using bone cements. While bone cements provide the initial fixation necessary for healing following surgery, bone cements often result in a very stiff overall structure, are prone to loosening with time, and can provoke tissue reactions.
Because of the disadvantages associated with the use of bone cements, xe2x80x9ccementlessxe2x80x9d or xe2x80x9cpress fitxe2x80x9d implants have been developed. Various procedures are used to affix a cementless or press fit implant to bone. In one procedure, the surface of the implant is coated with a porous material which allows the patient""s bone to grow into the pores, thereby mechanically fixing the implant to bone. In another procedure, the surface of the implant is coated with a material, such as hydroxyapatite, which can chemically bond the implant to bone. As a result of either procedure, the implant is biologically attached to the bone in that the patient""s own tissue eventually holds the implant securely in place, either mechanically or chemically, and the implant subsequently becomes a permanent part of the bone.
Typically, a cementless or press-fit implant requires immediate rigid fixation to the bone for a sufficient time period (e.g., at least six to twelve weeks) to assure mechanical attachment or chemical bonding to the bone. If the implant is not held rigidly, micro-motion occurs at the implant-bone interface. The result is a less stable fibrous tissue interface rather than the necessary, more stable, securely-fixed bony attachment. One method for holding the implant rigidly in the bone involves the use of an implant with a stem. The stem xe2x80x9cpress-fitsxe2x80x9d into the intramedullary canal of the bone (e.g., the femur) to hold the implant rigidly in the bone and thereby allows for an adequate mechanical attachment or chemical bonding for secure fixation.
A recognized problem with the use of an interference fit (press-fit) stem is that transfer of stress from the implant to the bone is abnormal. Instead of a normal loading of the bone primarily at the end of the bone near the joint surface, the bone is loaded more distally where the stem of the implant contacts and/or is affixed to the bone. This results in a phenomenon called xe2x80x9cstress shieldingxe2x80x9d in which the load (i.e., stress) bypasses or xe2x80x9cunloadsxe2x80x9d the end of the joint surface portion of the bone. As a result, the joint surface portion of the bone undergoes resorption, (i.e., the bone retreats from its tight fit around the prosthetic implant) thereby introducing some xe2x80x9cplayxe2x80x9d into the fit. This leads to weakening over a period of years, thus creating a potential for fracture or a loosening of the implant within the bone.
Various methods have been proposed to solve the aforementioned stress shielding problems. For example, U.S. Pat. No. 5,458,653 discloses a hip joint stem that has a biocompatible bioabsorbable polymer coating on selected locations of the stem. The polymer coating initially retards anchoring of the stem to the bone at the selected locations. When the polymer coating is absorbed into the body, bone ingrowth can occur in the spaces left behind by the polymer. In one version of this hip joint stem, the thickest part of the polymer coating is located at the distal end of the stem and the thinnest part of the polymer coating is located at the proximal end of the stem. In this arrangement of the polymer coating, the coating is absorbed progressively and gradually from the stem such that bone affixation progresses down the length of the stem to the distal end of the stem. U.S. Pat. Nos. 6,013,104, 5,935,172, 5,528,034, and 5,507,833 also disclose similar techniques in which a prosthesis includes bioabsorbable coatings or sleeves to control bone ingrowth.
Upon review of the foregoing methods that have been proposed to solve the problems associated with stress shielding in a prosthetic implant, it appears that U.S. Pat. No. 5,458,653 may describe the most sophisticated solution. Specifically, the method in U.S. Pat. No. 5,458,653 in which the thickness of the bioabsorbable polymer coating is varied along the length of the stem can provide for bone affixation that progresses down the length of the stem to the base distal end of the stem. Although this method could be a valuable tool for eliminating stress shielding in a prosthetic implant, it does have one significant disadvantage. While a manufacturer of this coated implant can provide an implant having a stem with a bioabsorbable polymer coating of varying thickness, it would be very difficult for a surgeon to precisely alter the thickness of the polymer coating on the coated implant or to apply a bioabsorbable polymer coating of varied thickness to the stem of an uncoated implant during surgery. Therefore, the orthopedic surgeon is essentially limited to an implant with a stem having a bioabsorbable polymer coating of predetermined thicknesses set by the manufacturer. This limits the surgeon""s ability to control bone affixation at different locations on the prosthesis.
Therefore, there is a need for an implantable bone prosthesis and associated surgical methods that provide an improved solution to the problems associated with stress shielding and that allow a surgeon to control bone affixation rates and final strengths at different locations on a prosthesis.
The foregoing needs are met by a prosthesis for implanting into a bone in accordance with the invention. The prosthesis comprises a body and a stem that extends away from the body. The stem has an outer surface and is dimensioned to be inserted into a cavity of the bone. A first coating including a bone ingrowth promoting material that promotes ingrowth of bone onto the stem and a bioabsorbable material that delays the ingrowth of bone onto the stem is disposed on a proximal portion of the outer surface of the stem. A second coating including the bone ingrowth promoting material and the bioabsorbable material is disposed on a distal portion of the outer surface of the stem. The ratio of the bone ingrowth promoting material to the bioabsorbable material in the first coating is greater than the ratio of the bone ingrowth promoting material to the bioabsorbable material in the second coating. By formulating the first coating with a higher ratio of the bone ingrowth promoting material to the bioabsorbable material, it is possible to implant the prosthetic implant in the bone such that the affixation of the bone to the proximal zone of the stem is faster and stronger over time than the affixation of the bone to the distal zone of the stem.
In another aspect of the invention, the first coating and the second coating may include different bone ingrowth promoting materials and different bioabsorbable materials. The bone ingrowth promoting materials and the bioabsorbable materials are selected such that the first coating promotes faster and greater bone ingrowth onto the stem compared to the second coating. As a result, it is also possible to implant the prosthetic implant in the bone such that the affixation of the bone to the proximal zone of the stem is faster and stronger over time than the affixation of the bone to the distal zone of the stem.
In yet another aspect of the invention, there is provided a method for implanting a prosthesis in a bone. The method involves applying a first coating including a bone ingrowth promoting material and a bioabsorbable material onto a proximal portion of a stem of a prosthesis and applying a second coating including the bone ingrowth promoting material and the bioabsorbable material to a distal portion of the stem of the prosthesis. The prosthesis is then inserted into a cavity in a bone such that the first coating and the second coating are placed in contact with the bone. By formulating the first coating and the second coating such that the ratio of the bone ingrowth promoting material to the bioabsorbable material in the first coating is greater than the ratio of the bone ingrowth promoting material to the bioabsorbable material in the second coating, it is possible to affix the prosthesis to the bone whereby the affixation of the bone to the proximal portion of the stem of the prosthesis is faster and stronger over time than the affixation of the bone to the distal portion of the prosthesis.
In still another aspect of the invention, there is provided another method for implanting a prosthesis in a bone. The method involves applying a first coating including a first bone ingrowth promoting material and a first bioabsorbable material onto the proximal portion of a stem of a prosthesis and applying a second coating including a second bone ingrowth promoting material and a second bioabsorbable material onto the distal portion of the stem of the prosthesis. The prosthesis is then located in a cavity of the bone such that the first coating and the second coating are placed in contact with the bone. By selecting the first and second bone ingrowth promoting material and the first and second bioabsorbable material such that the first coating promotes faster and greater bone ingrowth onto the prosthesis compared to the second coating, it is also possible to affix the prosthesis to the bone whereby the affixation of the bone to the proximal portion of the stem of the prosthesis is faster and stronger over time than the affixation of the bone to the distal portion of the stem of the prosthesis.
In a further aspect of the invention, there is provided a method for affixing a prosthesis to a bone. The method involves applying a first coating including a bone ingrowth promoting material and a bioabsorbable material onto a first portion of a prosthesis and applying a second coating including the bone ingrowth promoting material and the bioabsorbable material to a second portion of the prosthesis. The prosthesis is then located such that the first coating and the second coating are placed in contact with the bone. By formulating the first coating and the second coating such that the ratio of the bone ingrowth promoting material to the bioabsorbable material in the first coating is greater than the ratio of the bone ingrowth promoting material to the bioabsorbable material in the second coating, it is possible to affix the prosthesis to the bone whereby the affixation of the bone to the first portion of the prosthesis is faster and stronger over time than the affixation of the bone to the second portion of the prosthesis.
In still another aspect of the invention, there is provided another method for affixing a prosthesis to a bone. The method involves applying a first coating including a first bone ingrowth promoting material and a first bioabsorbable material onto a first portion of a prosthesis and applying a second coating including a second bone ingrowth promoting material and a second bioabsorbable material onto a second portion of the prosthesis. The prosthesis is then located such that the first coating and the second coating are placed in contact with the bone. By selecting the first and second bone ingrowth promoting material and the first and second bioabsorbable material such that the first coating promotes faster and greater bone ingrowth onto the prosthesis compared to the second coating, it is possible to affix the prosthesis to the bone whereby the affixation of the bone to the first portion of the prosthesis is faster and stronger over time than the affixation of the bone to the second portion of the prosthesis.
It is therefore an advantage of the present invention to provide a joint prosthesis that attaches to bone and has variable bone affixation rates at different locations on the prosthesis.
It is another advantage of the present invention to provide a joint prosthesis that attaches to bone and has variable final strengths over time at different locations on the prosthesis.
It is a further advantage of the present invention to provide a method for affixing a prosthesis to a bone wherein the prosthesis attaches to the bone with variable bone affixation rates at different locations on the prosthesis.
It is yet another advantage of the present invention to provide a method for affixing a prosthesis to a bone wherein the prosthesis attaches to the bone with variable final strengths over time at different locations on the prosthesis.
These and other features, aspects, and advantages of the present invention will become better understood upon consideration of the following detailed description, drawings, and appended claims.